The present invention relates to a method and to apparatuses for measurement of the fluorescence relaxation period of the radiation of a fluorescent substance whose fluorescence relaxation period is dependent upon at least one physical parameter, in which the substance is subjected to radiation that is repeated at intervals of time, the radiation producing fluorescence radiation which is advanced to a photoelectric receiver whose output signals are phase-sensitively rectified and integrated and influence the regulating parameter of a regulating circuit, whose regulating parameter is a parameter for the physical parameter.
A fiber optic temperature sensor is known in which the fluorescent radiation is excited by a sinusoidally modulated radiator. The signal that is obtained from the delayed fluorescent light is fed back to the control means of the modulator via a timing element. The frequency to be set in the self-exciting oscillation circuit is a function of the fluorescence relaxation period, and thus of all physical parameters that influence this period (German Disclosed Patent Application No. 3,202,089). If the output signal of the photoelectric receiver is integrated during the excitation phase and the relaxation phase of the fluorescence radiation, measures are required for suppression of the crosstalk of the excitation source and for control of the curve of the excitation over time. It is necessary for optical filters and electronic stray light compensation means to be employed.